The present invention relates to lithographic projection methods, systems, and apparatus and to products of such methods, systems, and apparatus.
The term xe2x80x9cpatterning structurexe2x80x9d should be broadly interpreted as referring to any structure or field that may be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, such a pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the projection beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask.
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference.
A first object table may be used to hold the patterning structure at a desired position in the incoming projection beam and to allow the patterning structure to be moved relative to the beam if so desired. For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and a mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
The term xe2x80x9cprojection systemxe2x80x9d should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin-stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (e.g. a wafer of silicon or other semiconductor material) that has been coated with a layer of radiation-sensitive material (resist). In general, a single substrate will contain a whole network of adjacent target portions that are successively irradiated via the projection system (e.g. one at a time). Among current apparatus that employ patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus (commonly referred to as a step-and-scan apparatus), each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. The projection beam in a scanning type of apparatus may have the form of a slit having a slit width in the scanning direction. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a scanning type of lithographic projection apparatus, positioning structures are used to move the tables during imaging. It is very important that these positioning structures will work with a high precision so that both tables will move synchronously during imaging and will not cause imaging errors. Positioning structures that are not working with the required precise synchronization may lead to high MSD (moving standard deviation) and high MA (moving average) synchronization errors for the tables. While both tables receive the correct, exactly time-synchronous movement profiles from a set point generator, what counts is the relative position error between the substrate and the mask table:
e=eWSxe2x88x92eRS/4xe2x80x83xe2x80x83[1]
wherein eWS equals the position difference between the substrate table and its set point, and eRS equals the position difference between the mask table and its set point.
FIG. 2 shows an example of the acceleration a of a substrate table during a scan in the Y-direction. To make an exposure scan, the substrate table has to move over a distance that is equal to the target portion length 11 plus two times the slit width 13 plus the travel length 7 during the required settling time. The settling time enables the position errors to decrease. The acceleration takes place as the table travels the acceleration distance 5, and the deceleration takes place as the table travels the deceleration distance 15. At the start of the exposure 17, the first point on the target portion to be exposed reaches the illuminated slit 13. After the exposure time Texp this first-exposed point leaves the slit 13. After a time period that equals the target portion length divided by the stage speed, the very last point 19 of the target portion leaves the slit. Together, these amounts add up to the mentioned scan length 9.
The average stage position error during the time a specific point of the target portion is in the illuminated slit 13 determines the shift in the radiation sensitive material on the substrate of this specific point (overlay). For every point x in the target portion, such a shift can be calculated, which is called the Moving Average (MA) error of this point x:                               MA          ⁡                      (            x            )                          =                              1                          T              exp                                ⁢                                    ∫                                                t                  X                                -                                                      T                    exp                                    /                  2                                                                              t                  X                                +                                                      T                    exp                                    /                  2                                                      ⁢                                          e                ⁡                                  (                  t                  )                                            ⁢                              xe2x80x83                            ⁢                              ⅆ                t                                                                        [        2        ]            
Here, Texp is the exposure time (which equals the slit width divided by the scan speed of the substrate table), e(t) is the relative substrate table/mask table position error at substrate level as function of the time, and tX is the time instant that point x is positioned at the lens center.
In addition to an average position error, the position error may have high-frequency variations during exposure, resulting in fading effects such as image non-sharpness or contrast loss. This effect is characterized by the Moving Standard Deviation (MSD), which equals the standard deviation of the relative position error during the exposure:                               MSD          ⁡                      (            x            )                          =                                            1                              T                exp                                      ⁢                                          ∫                                                      t                    X                                    -                                                            T                      exp                                        /                    2                                                                                        t                    X                                    +                                                            T                      exp                                        /                    2                                                              ⁢                                                                    [                                                                  e                        ⁡                                                  (                          t                          )                                                                    -                                              MA                        ⁡                                                  (                          x                          )                                                                                      ]                                    2                                ⁢                                  xe2x80x83                                ⁢                                  ⅆ                  t                                                                                        [        3        ]            
Both MA and MSD are calculated for every point in the target portion along the scanning direction Y, and the peak MA and MSD values in the target portion are used as performance indicators.
It has been proposed in U.S. Pat. No. 6,373,072 B1 incorporated herein by reference to provide a control system for the substrate and mask tables of a lithographic apparatus in which errors in the position of the substrate table are compensated for by their inclusion as a feed-forward control in the mask table control loop. Specifically, in such a system the substrate table error is lowpass-filtered, and the output of the filter is then added to the mask table set point. The output of the filter is also twice differentiated and multiplied by the mask table mass, and the resultant force is applied to the mask table. This proposed system is based on the realizations that the absolute positions of the mask and substrate tables are less important than their relative position and that it is easier to control the position of the mask than the position of the substrate. The latter condition applies because any absolute error in the positioning of the mask is multiplied by the magnification of the lens system before it adds up to the imaging error at the substrate surface. The magnification of the lens will be typically 0.25 or 0.2, so that any error at the mask will be four or five times smaller at the surface of the wafer.
Embodiments of the invention may include lithographic projection apparatus that have an improved synchronization and consequently a lower MSD and MA value.
A lithographic projection apparatus according to one embodiment of the invention includes a first object table configured and arranged to hold a patterning structure capable of patterning a projection beam of radiation according to a desired pattern and a second object table configured and arranged to hold a substrate. A positioning structure is configured and arranged to generate a force so as to move one of the object tables with respect to a projection system during an imaging operation. Processing circuitry is configured and arranged to read from a data storage device a compensation force value that corresponds to a value of a signal representing a position of said object table and to generate a force adjustment signal based on the compensation force value. The positioning structure is further configured and arranged to generate the force according to the force adjustment signal. The signal representing the position of an object table may be generated with a measurement system for measuring the position of the object table or may be a set point generated by a controller configured and arranged to control the movement of the object table.
A device manufacturing method according to one embodiment of the invention includes using a patterning structure held by a first object table to endow a projection beam of radiation with a pattern in its cross-section. The patterned beam of radiation is projected onto a target portion of a layer of radiation-sensitive material that at least partially covers a substrate on a second object table. This method also includes reading a compensation force value from a data storage device, where the compensation force value corresponds to a value of a signal representing a position of one of the object tables. A force adjustment signal is generated in response to the compensation force value, and a force is applied to the object table according to the force adjustment signal.
A measurement method according to a further embodiment of the invention includes controlling a positioning structure of a lithographic projection apparatus to move an object table of the lithographic projection apparatus in at least a first degree of freedom such that the object table moves with a constant speed in the first degree of freedom. This method also includes storing (e.g. in a data storage device) information relating to a value of a force exerted by the positioning structure to keep the object table moving with the constant speed in the first degree of freedom. For example, this information may be stored as a function of a corresponding position of the object table in the first degree of freedom.
Although specific reference may be made in this text to the use of apparatus and/or methods according to embodiments of the invention in the manufacture of ICs, it should be explicitly understood that such apparatus and/or methods have many other possible applications. For example, such applications may include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d, and xe2x80x9ctarget areaxe2x80x9d or xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms illumination radiation and illumination beam are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet (UV) radiation (e.g. at a wavelength of 365 nm, 248 mn, 193 nm, 157 nm or 126 nm), extreme ultraviolet (EUV) (e.g. having a wavelength in the range 5-20 nm), X-rays, electrons and ions.